The role of nerve muscle interactions in the development and differentiation of mammalian muscles of mastication and the patterning of neuronal organization has received little attention. However, much research has concentrated on the role of nerve-muscle interactions in limb muscle development. Previous studies in the limb have determined that motoneurons and the muscles they innervate are dependent on one another for their normal development, their maintenance and their physiological properties. Although valuable clues as to the role of nerve-muscle interactions in the development and maintenance of masticatory muscles can be gleaned from studies of limb muscle, the result of limb studies may not be totally applicable. The muscles of mastication differ from limb muscle in their embryonic origin, the origin of the sensory and motor neurons innervating them and in the uniqueness of the fiber types expressed in the adult. Moreover, an understanding of developmental processes involved in orofacial morphogenesis is required for an understanding of the adult form and function, and may provide valuable insight into what limitations might be encountered in modifying the adult form. Therefore, the focus of this proposal is to investigate the temporospatial development of the masticatory muscles and the trigeminal complex to enhance our understanding of nerve-muscle interactions during development. Two hypotheses are proposed to be tested during this project and focus on trigeminal central nervous system organization, masticatory muscle development and nerve-muscle interactions. The first hypothesis is that central components of the trigeminal system involved in the innervation of the muscles of mastication including motV and mesV, develop in distinct temporospatial sequences and in response to cues from their targets. The second hypothesis is that the adult phenotype of masticatory muscle fibers and the muscle fiber distribution within each jaw muscle are dependent on trigeminal nerve-muscle interactions. These two hypotheses will be tested in three specific aims that include in vivo and in vitro investigations.